Can anyone recommend the best size / weight shakey head jig to use with 5 inch rubber worms?

1/8 to 1/4 oz. is my favorite size.

I use 1/8 oz most of the time. I will go to 3/16 or 1/4 depending on depth and wind.

3/32 or 1/8 is a great overall weight.  The only time I go heavier is if I'm using the Buckeye Lures Spot Remover, then I will use the 1/4oz

You can use any size you want.  Practically speaking, weight is a function of how windy is it & how fast do you want to drop.  I generally use 1/4, yet differenty companies jigs weigh different even if they are labeled the same.  I think you have to tie it on to gauge the drop.